A valve of the kind specified is known from German Patent Specification DE 32 27 317 C2. In that valve the valve outlet can be closed by a piston which, when the valve is closed, fits positively into the conical outlet, which it seals. The piston is formed with an axial blind bore, from which channels extend radially and discharge on the outside of the piston. The outlet openings of the channels are so arranged that the channels are sealed by the inside wall of the outlet when the valve is closed. The piston is borne by an axially movable hollow spindle whose bore is connected on one side to the blind bore of the piston and on its other side to a drawing-off pipe for the booster steam. As soon as the piston is raised for the opening of the valve, pressurized steam flows into the gap produced between the piston and the outlet wall. A component flow of the steam flows through the channels of the piston and the bore of the hollow spindle into the drawing-off pipe.
The advantage of the known valve is that no additional valve is required for drawing off the booster steam, which is always reliably available at the same time as the valve is opened. However, a disadvantage of this valve is that it involves high manufacturing costs. Moreover, it has been found during the practical use of this Valve that sealing problems arise in the drawing-off of the booster steam. Lastly, the known valve does not ensure that an adequate volumetric flow of booster steam is available in the zone of water injection when the valve is opened.